Controller design for dynamic voltage restorer with harmonics compensation function

This paper describes the design of the control loop of a enhanced dynamic voltage restorer (DVR) controller which can handle power quality problems from not only voltage sag/swell but also harmonics. A novel controller, which has a feed-forward loop and two feed-back loops, is proposed. A feed-back loop is used to actively increase the damping component of LC filter system to keep the output voltage overshoot within allowable range, while a feed-forward loop is used to compensate the voltage drop due to the feedback loop. Another feed-back loop based on the digital repetitive control algorithm is implemented to compensate the periodic harmonics of load voltage, which may come from the source, power converter, and nonlinear load. The feasibility of the proposed DVR controller has been verified by the computer simulation results and also by experimental results on 5 kVA DVR system under the various source and load conditions. The both results reveal satisfactory performance not only against sag/swell but also against harmonics.

I. Introduction

Conventionally, Dynamic Voltage Restorer (DVR) has been used to improve power quality against the voltage sag or swell. However, there are many other problems that make the power quality worse, such as voltage harmonics, notch and the distortion by nonlinear load currents and by DVR itself. These kinds of harmonics and distortions may result in torque ripples in electric machines, saturation of transformer, abnormal losses of capacitors, and so on [1]~[2]. Recently, there are many researches to tackle these problems using the series compensators like SVC and DVR. The control strategies in these researches are selective harmonics compensation using d-q axis controller [3], resonant filter [4], fuzzy controller [5], and etc. However, these control methods have some disadvantages such as the complexity in the implementation of the controller, parameter sensitivity, and insufficient compensation under the nonlinear load condition. Besides, the conventional feed-back type controllers are prone to show the large voltage overshoot under the transient state. It has been shown that the voltage overshoot can be suppressed effectively by feed-back of the inverter current that actively increases the damping of the system [6]. However, such type of a controller has limited bandwidth due to the delay of the signal processing and it is difficult to use to reduce the higher order harmonic voltage distortion.